ARC-transient-grating maps
of photosynthetic pigments
for pulse-time delays, with
signal band-pass filtering at
880 nm ( 10 nm bandwidth).
Getty Images
Unraveling the Mysteries
of Photosynthesis
Photosynthesis is a biological process
that is both remarkably efficient
and somewhat mysterious. Scientists
don’t understand exactly how plants
absorb energy from photons and direct
it through living structures. For decades,
nonlinear optical four-wave mixing
(FWM) has been used to investigate
the process, but it hasn’t yielded a full
picture of energy transfer.
Now, a refinement on FWM methods has provided new information
about how energy is transferred within
photosynthetic organisms. Ian Mercer
of University College Dublin (Ireland)
and colleagues created a technique called
angle-resolved coherent (ARC) optical
wave-mixing that allows them to separate
coherently coupled electronic transitions
and energy transfers in an instantaneous
two-dimensional mapping (Phys. Rev.
Lett. 102, 057402).
Coherent optical FWM reveals information about the time scales of energy
transfer in complex molecular systems.
Because this technique uses three laser
pulses (the fourth wave is the signal
emitted from the sample), the timing
of the pulses can be chosen to enhance
sensitivity to energy transfers of interest. “There are various standard pulse
sequences that can be combined with
ARC,” Mercer explains.
A team from Lawrence Berkeley
National Laboratory (Calif., U.S.A.)
recently provided strong evidence that
a coherent coupling of electrons,
rather than more classical energy
transfer between electrons, may have
a role in photosynthesis (Nature
446, 782). Coherent coupling occurs
when molecules are very close and
their electrons’ quantum wave functions overlap,
so that
energy
may be
transferred
between
allowed quantum states of the net
molecule wave function. Coherent
electron coupling can be recognized
by the “quantum beating” of electronic dipoles.
Recognizing coherent coupling,
and differentiating between
this and other energy
transfers, isn’t easy.
Previous FWM
methods use tiny
beam waists (typically less than 100 µm),
partly because the intensities must be
high for the nonlinear effect, but also
because, if the interaction volume was
larger, the signal emission angle might
vary, producing an unwanted structure
in the far field.
Mercer and coworkers designed the
system to spatially separate the signal
beam. And the distribution of the signal
output at the detector provides information about whether the energy transfer
involves quantum beating or other
mechanisms.
For this system, they used a state-of-the-art hollow fiber laser, which
produces short broadband pulses and
high powers in the near-infrared. Two
of the co-authors, John Tisch and Jon
Marangos at Imperial College London
(England), led the fiber development,
and the technology was subsequently
used with the Rutherford Appleton
Laboratory’s Astra laser.
The fiber provides a broad
bandwidth and improves
beam stability. Tisch and
Marangos have been using
this technology to examine
the very fast-
est changes
in atoms and
molecules, on
sub-femtosecond
timescales. “This
technology is
ideally suited
to ARC map-
ping,” Mercer
says, “because
the colors of the
light are coherently related to each
other,” allowing a sensitivity to coher-
ent coupling between
molecular electrons
with different energies.
The ARC method pro-
vides a lot of informa-
tion with a single pulse,
to provide a snapshot
without the sample degrading.
Yvonne Carts-Powell ( yvonne@nasw.org) is a freelance science writer who specializes in optics and photonics.
